The present invention relates to a semiconductor device and more specifically to a semiconductor device in which MOSFETs (metal oxide silicon field effect transistors) are formed in the well regions of a semiconductor substrate maintaining high reliability, high degree of integration and high-speed operation.
FIG. 1 shows carrier concentration profiles of the conventional well structures. These profiles have been taught in a literature "Solid State Technology, Aug. 1984, pp. 123-131". FIG. 1(a) shows a well structure that is nowadays put into practice most generally, and in which the concentration profile is nearly uniform from the surface toward the direction of depth. In FIGS. 1(b) and 1(c), a high-concentration layer exists deep under the surface. This is called a retrograde well. Namely, a high-concentration layer is added to decrease the resistance of the well, in order to avoid a problem specific to the CMOS structure which contains both an n-channel MOSFET and a p-channel MOSFET, i.e., to avoid the parasitic thyristor effect. What makes a difference between FIG. 1(b) and FIG. 1(c) is that the high-concentration layer exists near the surface in FIG. 1(b) and exists deep under the surface in FIG. 1(c).
Generally, the concentration of well region of the MOS is intimately related to an initiation voltage that forms the channel when a voltage is applied to the gate electrode, i.e., intimately related to a threshold voltage V.sub.TH. The threshold voltage V.sub.TH increases with the increase in the concentration. The threshold voltage V.sub.TH also increases when a voltage (substrate bias) of a direction opposite to the source is applied to the well, since the depletion layer just under the gate extends in the direction of depth and the amount of fixed charge increases in the depletion layer. The rate of V.sub.TH increment when the substrate bias is applied is called substrate effect constant K. The substrate effect constant K increases with the increase of the concentration and deteriorates the circuit performance in an integrated circuit in which the potential of the well region cannot be fixed. In the retrograde well structure which contains a high-concentration layer, effects such as V.sub.TH and K that affects the MOS characteristics must be avoided.
In the example of FIG. 1(b) which contains the high-concentration layer near the surface, the threshold voltage V.sub.TH and the substrate effect constant K may increase greatly.
Another important technical assignment for fine MOSFETs includes a problem of soft error caused by a-particles. This becomes conspicuous particularly in the memories. That is, alpha particles emitted from radioactive elements, such as uranium or thorium contained in trace amounts in the package material, fall on the MOSFET to generate about 10.sup.6 electron-hole pairs in the semiconductor substrate to form noise charge which causes the memory to erroneously operate. This is called alpha-ray soft error. The high-concentration layer in the retrograde structure works as a potential barrier wall against the noise charge generated in a portion under the high-concentration layer, and prevents the noise charge from flowing into the drain of the MOSFET. However, the high-concentration layer does not exhibit the barrier wall effect for the noise charge generated over the high-concentration layer. To cope with this problem, therefore, the high-concentration layer should be formed as close to the surface as possible, so that the noise charge is generated in small amounts.
In the example of FIG. 1(c) in which the high-concentration layer is formed deep under the surface, generation of noise charge is not reduced in the above-mentioned sense, and the problem of alpha-ray soft error is not solved.